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Corolis Mass Flowmeters (Part 1 of 3) by David W Spitzer
Coriolis mass flowmeters use the properties of mass to measure mass.
In an analogy, when a fixed mass is rotating on a turntable, centrifugal force pushes the mass outward, but there are no forces pushing the mass in a plane tangent to the rotation.
However, if the mass is moving inwards or outwards from the center of the turntable, the radius of rotation changes, and a force (the Coriolis force) is produced in the tangential plane at right angles to the centrifugal force.
This effect can be readily experienced when riding a merry-go-round.
In a Coriolis mass flowmeter, the "rotation" is typically generated by vibrating tube(s) in which the fluid flows.
In a U-tube Coriolis mass flowmeter design, the fluid (mass) in the tubes flows away from and towards the axis of vibration, and a Coriolis force in the tangential plane is produced.
Fluid flows in opposite directions relative to the axis of vibration cause the Coriolis forces on the inlet and outlet halves of the U-tube to be in opposite directions.
These opposite Coriolis forces cause the U-tube to twist.
The amount of twist is proportional to the mass flow rate of fluid passing through the U-tube.
Other designs utilize different geometries to develop Coriolis forces.
Sensors and a Coriolis mass flowmeter transmitter are used to measure the twist and generate a signal proportional to the mass flow of the fluid.
The mechanical characteristics of vibrating tubes are affected by temperature, so the twist of the tube generated by a given Coriolis force will vary with temperature.
To maintain accurate mass flow measurement, Coriolis mass flowmeters generally require compensation for tube temperature or operation within a narrow temperature range.
Temperature compensation is typically implemented electronically in the Coriolis mass flow transmitter using a measurement from a temperature sensor located on the surface of the flow tube or in the flowmeter housing.
In many designs, suppliers offer an additional analog output that can represent temperature.
It should be noted that this output is the temperature used for compensation.
It is not the fluid temperature, but rather the temperature of the outer surface of the tube or the flowmeter housing.
Nonetheless, this temperature is often sufficiently close to the fluid temperature and may provide useful process information.
The frequency of the vibrating tube(s) is related to the density of the fluid in the tube.
The Coriolis mass flow transmitter typically analyzes the sensor signals to determine density of the fluid in the tube.
In many designs, suppliers offer an additional analog output that can represent the density of the process fluid.
Part 5: Measuring Gravity Flow: What Goes Up, Must Come Down by David W Spitzer
Previous columns have covered operation under vacuum and measuring downward liquid flow, so let's move on to gravity flow.
Most flowmeter specification sheets contain information regarding the flow range in which particular flowmeters are designed to measure accurately. Often, the maximum flow rate information is expressed as a maximum velocity and/or tabulated for different flowmeter sizes and design variations. The pressure drop exhibited by the flowmeter at various flow rates is often graphed or tabulated in another section of the specification sheet.
Excessive pressure drop across any flowmeter can limit the amount of flow that can pass through the flowmeter at operating conditions. However, pipes that flow via gravity typically operate at such low upstream pressures that most flowmeters that obstruct the flow can create a relatively large pressure drop that can hydraulically limit the flow rate to an amount that can be significantly less than the specified maximum capacity of the flowmeter. When this occurs, the upstream piping can potentially fill with liquid, flood upstream equipment and overflow. Therefore, it is extremely important that flowmeter sizing be performed carefully.
In general, when pressure drop limits flow, the upstream pressure can be increased and/or the pressure drop across the flowmeter can be reduced. It is not common to redesign a hydraulic process to accommodate a flowmeter, so in most instances, the pressure drop across the flowmeter will be reduced. This is generally achieved by increasing the size of the flowmeter, which in turn makes the flowmeter operate lower in its flow range --- often degrading flowmeter performance.
Which of the following units would be appropriate to measure a specific feed flow to a chemical reactor?
Liters (gallons) per unit time
Kilograms (pounds) per unit time
Cubic meters (feet) per unit time
Standard cubic meters (feet) per unit time
Commentary
Chemical reactions occur using the mass of the reactants so measurements that accurately reflect that mass would be appropriate. Liters (gallons) and cubic meters (feet) are units that reflect the volume of the reactants --- not their mass. Answer A and Answer C are not correct.
Kilograms (pounds) is a mass unit. Answer B is correct.
A standard cubic meter (foot) represents a mass because it describes the size of a specific fluid at known conditions. Answer D is also correct.
Additional Complicating Factors
Flowmeters such as Coriolis mass and thermal flowmeters actually measure mass flow.
Other flowmeters measure or infer velocity from which the mass flow can be calculated and displayed. Observing a display in mass units does not provide information as to whether the flowmeter is a mass flowmeter or not. As such, the observer does not know whether varying process conditions can introduce additional measurement error --- even during normal operation.
In addition to over 40 years of experience as an instrument user, consultant and expert witness, David W Spitzer has written over 10 books and 450 articles about flow measurement, level measurement, instrumentation and process control. David teaches his flow measurement seminars in both English and Portuguese.
Spitzer and Boyes, LLC provides engineering, technical writing, training seminars, strategic marketing consulting and expert witness services worldwide.
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